Electroluminescent image device



Aug. 15, 1961 F. KOURY ELECTROLUMINESCENT IMAGE DEVICE Filed April 11,1957 FREQUENCY (CPS) IN V EN TOR. FREDER/C KDURY VOL T A GE UnitedStates Patent 2,996,623 ELECTROLUMINESCENT IMAGE DEVICE Frederic Koury,Lexington, Mass, assignor, by mesne assignments, to Sylvania ElectricProducts Inc., Wilmington, Del., a corporation of Delaware Filed Apr.11, 1957, Ser. No. 652,162 3 Claims. (Cl. 250-213) This inventionrelates to electroluminescent image-producing devices, and especially tothose in which there is an array of parallel conductors on one side ofan electroluminescent layer, and a similar array oriented in a differentdirection on the other side.

When a proper voltage is applied between a conductor of one array and aconductor of the other, the spot where one conductor crosses the otherWill luminesce, and thus an image, moving or still, can be produced onthe electroluminescent device by applying potential to particularconductors at particular times. Such devices are known to'the cross overpoint, because capacity effects between conductors tend to cause otherpoints along each con-,

ductor to lurninesce, thus providing a sort of halation around eachactive point and tending to make the image unclear.

The voltage across the electroluminescent layer is, how: ever, greaterat the cross-over point than at the other luminescing points, and I havediscovered that electroluminescence at the additional points can beprevented or reduced by keeping the voltage at the cross-over point lowenough to insure that the voltage at the additional points will be belowthe threshold voltage for electroluminescence.

The voltage across the layer at the cross-over point will then be so lowthat the luminescence at that point will be reduced, but theluminescence there can be raised by increasing the frequency of theapplied voltage.

Thus by operating at a voltage-low enough to prevent luminescence atnoncrossover points, and increasing the frequency at the same time, Iobtain bright luminescence at the desired point, without luminescence atundesired points.

Other objects, features and advantages of the invention will be apparentfrom the following specification tafkfier;1 in connection with theaccompanying drawing in w c FIGURE 1 is a top view in perspective of adevice according to the invention;

FIGURE 2 is a graph of the brightness in foot lamberts against frequencyin cycles per second, and one embodiment of the invention; and

FIGURE 3 is a graph of relative brightness against voltage for the sameembodiment.

In FIGURE 1, the glass base plate 1 carries an array A of narrowparallel conductors 2 spaced apart and insulated from each other. Overthese conductors and in close contact therewith is a layer of phosphorand dielectric material 3 and over said layer is another array B ofnarrow parallel conductors 4, spaced apart and insulated from eachother, the conductors 3 being oriented in a plane parallel to the planeof the conductors 2 but said conductors 3 being at an angle with saidconductors 2.

The dielectric is preferably a ceramic material such as shown in anapplication of Richard M. Rulon, Serial No; 365,617, filed July 2, 1953,and the phosphor used can also be a phosphor such as used in saidapplication for example, a zinc sulfide activated with copper, lead andchlorine.

The conductors in at least one of the two arrays are preferably of atransparent conductive material such as the now well-known tin chlorideor the like. Such materials and the methods of application are shown insaid copending application of Richard M. Rulon. The tin chlorides may beapplied to the glass plate 1 and the phosphor-dieletric layer 3 througha mask to provide the series of parallel conductors, or the coating maybe applied to the entire top surfaces of the glass layer 1 and thephosphor dielectric layer 3 and then removed from those parts of saidsurfaces on which it is not desired. Methods of removing parts of suchcoatings are shown, for example, in my copending application, Serial No.631,131, filed December 28, 1956. The word top is here used merely todenote the surfaces which appear as top surfaces in FIGURE 1, but ofcourse they would not be the top surfaces if the device of FIGURE l wereturned over. Connections to the various conductors 2, 4 can be made bymethods well-known in the art.

In FIGURE 2, the electroluminescent brightness of a device such as shownin FIGURE 1 is seen to rise sharply with an increase of frequency andalso to be higher for higher voltage. FIGURE 3 shows, however, thatthere is a threshold voltage E below which no electroluminescenceoccurs. I have found that this threshold voltage is substantially thesame for all frequencies. Thus voltage which is considerably below thethreshold value for one frequency will be below it for all frequencies.

Accordingly a voltage V can be selected which is above the thresholdvoltage E, and this voltage can be applied between a conductor 2a inarray A and a conductor 4a in array B, by connections 5, 6 from A.C.voltage source V. The cross-over point C will then show electroluminescence.

Due to capacity eifects between the wires at points such as D, whereeither of the live conductors to which voltage is directly appliedcrosses over an electrically floating conductor to which no potential isdirectly applied, but which provides a capacitative path joining the twolive conductors, a voltage will exist between the live conductor and thefloating conductors, through a series of capacitors. There willgenerally be two or three such capacitive connections between the liveconductors, and accordingly the voltage between the line conductor and afloating conductor will generally be less than half the voltage betweenthe live conductors.

Accordingly it is possible to have the voltage between the two liveconductors above the threshold voltage, and the voltage with respect toother conductors below the threshold voltage. Cross-over point C willthen have voltage enough to emit light, whereas points D will not. Thelight emission will then be confined to the crossover point.

The light emission at point C will, however, be dim because of the needto keep the voltage at point D below the cross-over value. However, thefrequency can then be increased until the desired brightness isobtained.

In that way, the brightness of the cross-over point C will be increased,but not that at the point D, because the voltage at the latter pointswill be below the threshold voltage. If the threshold voltage B weresubstantially independent of frequency, an increase in brightness of Cwith frequency would be accompanied by an increase in brightness of Dalso, and the device would be ineffective. However, I have discoveredthat the threshold voltage is substantially independent of frequency,and that my device is quite effective.

In one specific example, the base plate 1 was of glass two inches squareand about .070 inch thick. The conductors 2, 2, and 4, 4 were oftransparent conductive film of tin chloride, as previously mentioned,and the electroluminescent layer was about 0.002 inch in thickness, andcomposed of a copper-activated zinc sulfide, containing lead andchlorine, as in US. Patent 2,728,730, issued December 27, 1956, to KeithH. Butler, embedded 3 in a ceramic material as shown in copendingapplication Serial No. 365,617, filed by July 2, 1953, by Richard M.Rulon.

A sine-wave alternating voltage of about 300 volts R.M.S., at about 4000cycles per second was applied between a conductor of array A and aconductor of array B, and point C glowed brightly, whereas points suchas D did not glow at all.

If the voltage were raised to 500 volts, points D glowed also, and ifthe voltage were reduced to 300 volts at 60 cycles per second, the spotat C Was dim. The use of a rfrequency of several hundred cycles persecond, for example 500 cycles per second, would improve the brightness.

In some cases, it may be desirable to arrange the two arrays in polarcoordinate form, that is, one array can be made up of conductorsarranged in a series of concentric circles, and the other in a series ofradial lines emanating from a center in register with that of thecircles.

A layer of dielectric material, without phosphor, can be used on eitheror both sides of the electroluminescent layer, if desired. Thedielectric material can be, if desired, the same used in thephosphor-dielectric layer 3, and one at least of the added dielectriclayers, should be of a light-transmitting material.

What I claim is:

1. An electroluminescent image display device comprising a layer ofelectroluminescent phosphor embedded in a dielectric material, a firstarray of parallel conductors on one side of said layer, a second arrayof parallel conductors on the other side of said layer, the conductorsin said second array being at an angle to those in the first array, andmeans for supplying an alternating voltage to a selected conductor insaid first array and a selected conductor in said second array, the fullvoltage of said means being above the threshold voltage forelectroluminescence in said layer, in order to provideelectroluminescence at the point where one of said selected conductorscrosses the other, but being low enough to insure that the lower voltageis below the threshold voltage at points where a selected conductor inone array crosses a non-selected conductor in the other array, thefrequency of said alternating voltage being at least several hundredcycles per second.

2. An electroluminescent image display device comprising anelectroluminescent layer, a first array of parallel conductors on oneside of said layer, a layer of dielectric material on the other side ofsaid electroluminescent layer, a second array of parallel conductors onsaid dielectric layer, the conductors in said second array being at anangle to those in the first array, and means for Supplying analternating voltage to a selected conductor in said first array and aselected conductor in said second array, the full voltage of said meansbeing above the threshold voltage for electroluminescence in said layer,in order to provide electroluminescence at the point where one of saidselected conductors crosses the other, but being low enough to insurethat the lower voltage is below the threshold voltage at points where aselected conductor in one array crosses a non-selected conductor in theother array, the frequency of said alternating voltage being at leastseveral hundred cycles per second.

3. The device of claim 1 in which there is a layer of dielectricmaterial between one of said arrays of parallel conductors and the layerof electroluminescent phosphor embedded in dielectric material.

References Cited in the file of this patent UNITED STATES PATENTS2,660,686 Putnam Nov. 24, 1953 2,698,915 Piper Ian. 4, 1955 2,837,660Orthuber June 3, 1958 FOREIGN PATENTS 717,169 Great Britain Oct. 20,1954 OTHER REFERENCES Bramley et 211.: Review of Scientific Instruments,June 1953, vol. 24, 471-472.

